Among all computing and networking security issues, the most important cause of concern does not come from intrusions, but from the widespread proliferation of viruses. Viral infections represent the great majority of all security incidents.
Virus Protection
Virus protection for large organizations has become more and more complex and difficult because of:                the combined use of heterogeneous systems and practices,        the widespread use of distributed or client/server systems, and the free exchange of data files via network sharing, e-mail, Internet . . . .        
Until recently, viral infections threatened only data residing on storage media, such as hard drives and floppy disks. However, with the emergence of macro viruses, the threat has spread to applications. Most organizations are not aware of this level of penetration and are not organized to manage and prevent virus attacks. An effective virus protection software must prevent infections rather than simply treating them after they have already occurred. Anti-virus solutions need a uniform plan, with a centralized control, automated virus signature updates, and support for multiple platforms, protocols, and file types.
Computer Viruses
A computer virus is any program created to reproduce itself. A virus reproduces itself by attaching itself to programs, files, or even to boot sectors of disks. A virus is activated when the infected file or disk is opened or accessed. Once a virus resides in a memory, it can attach itself to the next file or disk accessed, and so on. A virus may be designed to do harm. A virus may also have unintended consequences by overwriting important computer information and by causing costly inconveniences to users and network managers. There are four general types of computer virus:                File Viruses (including macro viruses), which are attached to files;        Boot sector Viruses in which the boot sectors of floppy or hard disks are infected;        Master Boot Record (MBR) Viruses which infect the disk master boot record; and        Multi-partite Viruses that are a combination of a file virus and a boot sector virus.Virus Disguises        
Viruses need to avoid detection in order to succeed in corrupting target computers. Simple viruses, with easily detectable signatures are giving way to more sophisticated virus types:                Polymorphic Viruses: they change their signature, or profile, each time they are activated so that a fixed signature filter will miss them.        Stealth Viruses: they attempt to hide their presence by intercepting interrupt services and by feeding back false information to anti-virus products and end users.        Encrypted Viruses: they are delivered within an encrypted file and are undetectable by a simple anti-virus.Sources of Infection        
Every improvement in network and communication technologies opens new avenues through which viruses can infect your system. Most of former viruses were boot sector viruses, in which the boot sectors of floppy or hard disks were infected.
Macro Viruses
As stated earlier, the creation of macro viruses has changed this environment dramatically. A macro virus is a set of instructions comprising powerful macro routines initially designed for word processing and spreadsheet applications. These macro languages enable a myriad of useful functions which can be imbedded into a document and which can be executed when the document is opened for view or use.
Internet
With the exploding development of the Internet, viruses have catastrophic possibilities. The Internet introduces two different virus threats.                The first threat is caused by the download of files comprising viruses when these files are browsed or transferred using for instance FTP (File Transfer Protocol) routines. Public shareware (shared software) and executable routines of all types, including formatted presentations, are a growing source of virus infection. Furthermore, new Internet virus threats are beginning to appear in the form of malicious JAVA and Active-X applets.        The second threat comes from electronic mail (e-mail). Most Internet e-mail systems provide a very rich capability to attach formatted documents to mail sent over the network. These e-mail messages can be broadcast to individuals or groups of individuals with the simple stroke of a key! Infected documents or files can flood a corporate network through gateways and mail servers. As networking, telecommunications, remote access, message systems supporting attachments of all kinds become more and more common, viruses will exploit these new electronic pathways to attack systems that were heretofore unreachable.Groupware Complications        
A third trend in networking also exacerbates the virus threat: the trend towards the deployment of Groupware applications such as Lotus Notes, Microsoft Exchange, Novell Groupwise, . . . .
Since the active and repeated sharing of documents over the network is at the core of these applications, they represent a fertile ground for the deployment of macro viruses.
A Groupware application not only acts as a repository for shared documents, but, due to its collaborative function, it simultaneously broadcasts files to associated work groups. The broadcast of files significantly multiplies the possibility of accidentally deploying mail infected by attached macro viruses and makes Groupware protection a high priority.
Symptoms of Virus Infection
Most viruses attempt to remain undetected as long as possible to extend their destructive influence. Therefore, most viruses do not produce any recognizable profile or signature that would allow to trap them by scanning the software. However, viruses perform actions that do not look like normal computer operations or user operations. These abnormal actions can be detected by intelligent anti-virus software. Fortunately, many viruses have telltale symptoms and may inadvertently give off signals that can alert users and virus protection software to their presence.
Some of these symptoms include:                Increase in byte length of files,        Alterations of a file's time stamp,        Delayed program loading or activation,        Reduced performance,        Lower system resources, available memory, disk space,        Bad sectors on floppies and hard drives,        Strange or non-standard error messages,        Non-standard screen activity, display fluctuations,        Program inoperability (failing to execute),        Incomplete or failed system boots, and        Uninitiated drive writes.Anti-virus Software OverviewDetecting a Virus        
Viruses are becoming increasingly sophisticated and, as such, can defeat simpler, single dimension software packages. To be effective, the anti-virus software must include special-purpose, distributed applications. Applications can detect viruses using five distinct methods:                Signature Scanning: This method compares the content of files against a database of virus signatures. This method requires frequent updates of the database to ensure the identification of new and changing signatures.        
Integrity Checking: This method compares the profile of current files and disk areas against an archived snap shop of these same items. The detected differences may indicate the presence of a virus. Check summing is the most common type of integrity checking. Unfortunately, integrity checking is generally not effective against modern stealth viruses, so further detecting means are needed.
Heuristic Analysis: An artificial intelligence monitors virus-like behavior, such as trapping certain interrupt services or attempting unlikely actions such as reformatting the hard disk.
Polymorphic Analysis: Polymorphic viruses are difficult to detect because they constantly change their look, particularly when they are encrypted or when they use stealth techniques to hide their presence. A polymorphic analyzer will move any suspect file to a separate, protected, location in the computer and will execute it there to see if it exhibits any virus-like behavior.
Macro Virus Analysis: A specifically designed anti-virus software detects macros in files and tests them before execution.
Archived and Compressed Files
In addition to the support of these five types of virus analysis, an effective anti-virus system must also be able to scan archived and compressed files. Zip (or Pkzip) and Microsoft Compression are the most common tools for archiving and compressing a file. A virus can hide inside a compressed archive, and can remain dormant or unnoticed until the infected file is extracted and released into a system. The minimum for an efficient anti-virus system is to be able to scan most current types of archives to identify viruses stored within the files they contain.
Frequency of Database Signature Update
Finally, the ability of a virus software to prevent virus attacks is determined by its ability to maintain an updated virus signature database. Any anti-virus software must have an associated, easily accessible Web site, or some other online source of information, where regular virus database updates can be retrieved. Products that automate this update process by using an Internet connection to regularly collect new information have a clear advantage in this regard.
Real Time and Scheduled Virus Scanning
Most anti-virus software can perform a scan of a computer in order to detect and possibly treat the viruses found at that time. This process is called scanning. Scanning a computer for viruses can occur:                at regular intervals under the control of a scheduler, or        as an on-demand operation manually executed, or        as an event-activated operation (usually in response to some recognizably “illegal” behavior by a potential virus).        
In addition, viruses can be detected in real time, when they are received. This capability is important because if viruses can be detected when they attempt to enter within a system (computer, data repository, server . . . ), then it is possible to prevent them from corrupting other files. Oftentimes, a scheduled scan may occur after a virus has already entered within a computer and has corrupted other files. Obviously, the earlier a virus can be detected, the better.
To be truly useful, an anti-virus software must have the ability to perform all types of scans.
Certificate
A Certificate is a structure that contains a public value (i.e. a public key) associated with an identity. For instance, within a X.509 Certificate, the public key is bound to a “user's name”. A third party (a Certificate Authority) attests that the public key belongs to the user. A X.509 Certificate is a very formal structure and comprises different elements:                Subject: This is the “user's name” (the Subject can be any identity value).        Issuer: This is the name of the third party that has issued/generated the certificate. This third party is the Certificate Authority (CA).        Public Key Value: This is the public key of a public/private key pair. An associated field defines the public key algorithm that must be used, for instance a RSA, Diffie-Hellman or DSA public key.        Validity: Two fields are used to define the period of validity (valid from date 1 and valid to date 2).        Serial Number: This field provides a unique Certificate serial number for the issuer.        Signature: The signature is an encrypted digest generated by the Certificate Authority (CA) for authenticating the whole certificate. The digest results from the hashing of the Certificate. The digest is encrypted using the CA private key. The encrypted digest which is the signature, “certifies” that the Subject is the “owner” of the public and private keys.Certificate Verification        
The Certificate needs to be verified to ensure that it is valid. This is a quite complex process. The verification by an end user of a Certificate comprises the checking of the following elements:                Valid (or any) Subject and Issuer names are defined in the Certificate.        The Certificate is not expired (checking of the Validity period field).        The Certificate has not been revoked (this may be determined by obtaining a current Certificate Revocation List from the CA).        The signature on the Certificate is valid (the signature is not verified by using the Certificate's public key but by using the CA public key).        
The method for validating the signature is quite simple, and comprises the steps of:                extracting the issuer's name (CA name) from the Certificate;        locating the issuer's Certificate (CA Certificate) or the issuer's public key (CA public key).        checking that the end user's Certificate signature was generated by the issuer (CA) using the issuer's public key (CA public key).        
Certificates are generated by a Certificate Authority (CA). Two main methods can be used:                Centralized Generation: The private/public key pair is generated by the end user (defined in the subject field of the Certificate). The public key is directly provided by the end user to the CA software to create a Certificate. The Certificate can be provided to another end user via any suitable channel. The channel does not have to be secure because a Certificate is a self protecting structure (given the CA's signature).        Distributed Generation: The private/public key pair is generated by the end user. The end user requests the CA to build a Certificate including the end user public key. The public key is then sent to the CA for certification. If the request is valid then the CA returns a Certificate associating the user identity with the user public key to the end user.        
Of course these two methods can be combined in any system, because trusted CA keys are generated by the Certificate Authority (CA).